In mobile communication networks, the radio base stations (RBS) are increasingly designed in such a way that the air interface is physically separated from the baseband signal processing. In practice, this means that radio equipment controllers (REC), which perform the baseband processing, are arranged at one location while the radio equipments (RE) are arranged at locations remote from the RECs, often on antenna masts, rooftops, roadsides, etc. In a distributed RBS, an RE is also referred to as a remote radio unit (RRU). This configuration allows the bulk of the processing to be performed remotely in a centralised fashion, which simplifies the design of the radio site. In effect, it enables all RECs to be located in a single physical site within a controlled environment. This kind of system architecture with centralised processing units is often referred to as Centralised Baseband (CBB), but is also often referred to as Centralised Radio Access Network (C-RAN) or baseband hotel.
In a distributed RBS or CBB, an REC is connected to REs via a backhaul network, which is typically an optical fiber network. The backhaul network must comply with the necessary requirements for data rates, bit error rate (BER), delay and delay variation. One manner of achieving this is to use the Common Public Radio Interface protocol or CPRI defined in “CPRI Specification V5.0 (2011 Sep. 21)” available at http://www.cpri.info/spec.html. CPRI is a layer 2 protocol which defines the signalling and control operations for transmitting digitised radio signal samples from REs to the processing units and vice-versa. Providing that the bandwidth, maximum delay, delay variation and BER requirements are met, CPRI can be run over different physical media, such as optical fibers, copper wires or even microwave links.
As each optical fiber in the backhaul network may be used to carry traffic from multiple mobile subscribers, it is essential that any failure in the backhaul network is detected promptly and communication restored with minimum delay.
Conventional fiber monitoring systems use reflectometry techniques to detect faults by transmitting test signals through a fiber and analysing the reflections. They are generally stand alone systems which can be customised to interact with an operator's Operations, Administration and Maintenance (OAM) system. For example, it is known to provide a manually operated fiber monitoring system in an access network that is configured as a passive optical network (PON), where the fiber condition is tested after a customer has complained. A fiber monitoring system operated in this way is not suitable for mobile backhaul optical fiber networks, because the downtime caused by a fiber fault can affect many mobile subscribers and cause substantial losses for the operator.
U.S. patent application No. US 2011/0311220 A1 and U.S. Pat. No. 7,738,787 both describe optical transmission line monitoring apparatus that use optical time domain reflectometry (OTDR) to continuously poll the status of each optical fiber in turn. Whilst these arrangements may react faster than a manually operated monitoring system, they are subject to scalability problems, as the time needed to poll all fibers increases with each additional managed fiber. A reduction in the total testing time can only be achieved by reducing the sampling rate for each fiber and thus at the expense of OTDR accuracy.
A further fiber monitoring arrangement and method is described in WO 2012/087205. In this arrangement a test procedure is triggered by an alarm received from a terminal device as a result of non-standardized performance parameters measured as part of an Optical Transceiver Monitoring (OTM) arrangement. The detection trigger is based on hard thresholds, e.g. absolute received power levels, which requires careful control of the operating conditions of such a system. Optical power measurements made using the same equipment, but at different operating temperatures can differ substantially.